By Dr. Leonard Perry, Extension Professor
University of Vermont
Fragrance, or scent, in plants was important historically in gardens, particularly in "grandma's garden" and Victorian gardens. Once again this trait of plants is becoming important in gardens, especially "cottage gardens."
Fragrance is an elusive quality, in that it may be fleeting and change over time. The human nose detects scents in small quantities (often parts per billion). Scents are actually the reaction of certain cells in the nose to volatile compounds emitted by essential oils in plant parts. These oils may come from roots, stems, leaves, or most commonly from flowers.
Scents are usually described in relation to everyday items with an odor, such as spices, flowers, fruits, and even unpleasant ones, such as perspiration. Scent is subjective and is described by each person as either good or bad. Whether you feel a scent is good or bad depends on personal likes, closeness of the scent, or emotion. What smells pleasant at a distance may be overpowering at close range. Scientists have described several dozen emotional responses to flower scents.
Scents actually have a function, usually for pollination by insects. The lighter colors of whites, pinks, and yellows have a pleasant but faint fragrance if any. These colors usually attract moths and butterflies for pollination, insects that see rather than smell.
Composite (daisy) and umbel (Queen Anne's lace, for instance) flowers often smell unpleasant, as these odors attract flies for pollination. Self-pollinating flowers that need no insects for pollination, or flowers attracting bees by sight rather than smell, often have no fragrance. New highly bred cultivars of flowers, often annuals, likewise may not have fragrance.
Some plants may use scents as protection from insects, or as protection from drought in hot, arid climates. For the latter, the thick volatile compounds we smell are what provide a protective layer around leaves. The old English custom of covering brick walls with sprigs of rosemary for cooling has been supported by modern research. Rosemary has 74 times the cooling effect of fresh air (thyme has 68 times the cooling effect, lavender 60 times).
In historic times, lack of sanitation whether from lack of daily bathing to lack of proper garbage disposal, led to many foul odors. Herbs known as "strewing herbs" were used to mask room odors by strewing or scattering about the floor to emit nice smells when walked upon.
Plants, and particularly herbs, were used to either cover body odors as perfumes or provide medicinal scents. Herbs were worn on the body, clothes, or carried as pomander balls. These uses have been supported by modern research. Cinnamon oil kills typhoid germs in 12 minutes. Other essential plant oils kill these germs in less than 50 minutes. Attar (essential oil) of roses has seven times the antiseptic (germ killing) strength of carbolic acid, while oil of thyme has 12 times the strength.
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